1. Field of the Invention
The present invention relates to a supercharged engine, and more particularly to an engine having a supercharger of a type in which the intake air is compressed by the pressure of the exhaust gas before it is introduced into the combustion chamber.
2. Description of Prior Art
Hithertofore, proposals have already been made of an engine supercharger which utilizes the pressure of the exhaust gas produced in the engine exhaust passage for compressing the intake air before the intake air is introduced into the combustion chamber. This type of supercharger is considered as being advantageous over turbo-superchargers because it can provide a higher supercharging effect under a low speed engine operation. A supercharger of this type generally includes a rotor having a plurality of mutually separated, axially extending gas passages and a casing supporting the rotor for rotation about an axis of rotation. The casing is provided with exhaust gas inlet and outlet openings and intake gas inlet and outlet openings which are located at opposite axial ends of the rotor. The arrangements are such that the intake air is drawn into the gas passages through the intake gas inlet opening and compressed by the pressure of the exhaust gas introduced into the gas passages through the exhaust gas inlet opening. As the rotor rotates, the gas passages are sequentially opened to the intake gas outlet opening so that the intake gas is forced by the exhaust gas to flow into the intake passage communicating with the intake gas outlet opening. Thereafter, the passages are opened to the exhaust gas outlet opening so that the exhaust gas is allowed to flow into the exhaust passage communicating with the exhaust gas outlet opening. The passages in the rotor are then scavenged by the air drawn from the intake gas inlet opening and passed through the passages to the exhaust gas outlet openings. Thus, it is required that the exhaust gas inlet opening and the intake gas outlet opening be located axially opposite to each other with respect to the rotor. An example of such a supercharger is disclosed by Japanese patent publication No. 38-1153. The supercharger disclosed by the Japanese patent publication is of a type wherein the exhaust gas inlet and outlet openings are located at one axial end of the rotor and the intake gas inlet and outlet openings are located at the other axial end so that the exhaust gas and the intake gas change their flow directions in the gas passages. As disclosed in detail by the Japanese magazine "Nainen-Kikan (Internal Combustion Engines)" Vol. 15, No. 179, 1976 June issue, there is also known a so-called through-flow type wherein the gas inlet and outlet openings are arranged so that the exhaust gas and the intake gas flow axially through the gas passages without changing the flow directions.
The supercharger of the aforementioned type is considered as being particularly suitable for diesel cycle engines, but can of course be used in gasoline engines as well.
In an engine having the aforementioned type of supercharger, the rotor of the supercharger is located in the engine exhaust passage, providing a resistance to the exhaust gas flow. Further, the exhaust gas in the passages of the supercharger rotor is opposed by the pressure of the intake air at the intake air outlet opening. As a result, the pressure of the exhaust gas in the exhaust passage upstream of the supercharger is maintained at a high level, providing a substantial back pressure to the engine exhaust port. Thus, the exhaust gas pressure at the engine exhaust port is markedly higher than the intake pressure at the engine intake port, resulting in an increased pumping loss.
It may be possible to decrease the pumping loss by relieving a part of the intake air in the intake passage downstream of the supercharger to the intake passage upstream of the supercharger to thereby decrease the intake air pressure opposing the exhaust gas in the passages of the supercharger rotor. However, such an intake air pressure relief is not recommendable under heavy load engine operation because a decrease in the supercharging pressure will cause a decrease in the intake air charge resulting in insufficient engine output and poor fuel economy. Further, this will also cause smoke in the exhaust gas in case of a diesel engine.
The intake air pressure relief further causes problems even under engine operation other than the heavy load condition. In the aforementioned type of supercharger, it is possible that the exhaust gas drawn from the exhaust gas inlet opening to the passages of the supercharger rotor is passed to the intake air outlet opening to be mixed with the intake air. This phenomenon may be utilized as a measure for providing exhaust gas recirculation, which is generally adopted for suppressing nitrogen oxides in the exhaust gas. However, when the intake air in the intake passage downstream of the supercharger is partly relieved to the intake passage upstream of the supercharger, the exhaust gas which has previously been mixed with the intake gas is also passed to the intake passage upstream of the supercharger. Such processes are recycled and the exhaust gas content in the intake air is ultimately increased to an extent that smooth engine operation is disturbed. Further, an increase in the exhaust gas content in the intake air causes an increase in the supercharger rotor temperature because the intake air can no longer cool the rotor to a satisfactory level.
It should further be noted that the exhaust gas recirculation effect accomplished by the exhaust gas which has passed through the passages in the supercharger rotor to the intake air outlet opening is, in general, insufficient in amount. It may be possible to increase the amount of recirculated exhaust gas by appropriately controlling the rotating speed of the rotor. However, an increase in the recirculated exhaust gas causes an undesirable increase in the temperature of the rotor. Further, where the supercharger is provided with a waste gate valve for relieving the exhaust gas pressure upstream of the supercharger to the exhaust passage downstream of the supercharger, for preventing the exhaust gas pressure from being increased to a dangerous level, the operation of the waste gate valve may cause a decrease in the exhaust gas pressure to a level wherein the amount of the recirculated gas is undesirably decreased.